LMC1982 Datasheet, PDF (9/14 Pages) National Semiconductor (TI) – Digitally-Controlled Stereo Tone and Volume Circuit with Two Selectable Stereo Inputs

Application Information (Continued)
if the total emitter current exceeds 5 mA. Thus, maximum
output voltage can be increased and much lower distortion
levels can be achieved using load impedances of at least
25 kΩ.
INPUT IMPEDANCE
The input impedance of pins 4, 5, 24 and 25 is defined by in-
ternal bias resistors and is typically 50 kΩ.
The SELECT IN pins have an input impedance that varies
with the BASE and TREBLE control settings. The input im-
pedance is 100 kΩ at DC and 19 kΩ at 1 kHz when the con-
trols are set at 0 dB. Minimum input impedance of 30.4 kΩ at
DC and 16 kΩ at 1 kHz occurs when maximum boost is se-
lected. At 10 kHz the minimum input impedance, with the
tone controls flat, is 6.8 kΩ and, with the tone controls at
maximum boost, is 2.5 kΩ.
FIGURE 2. Input and Mode Select Circuitry
DS011028-6
EXTERNAL SIGNAL PROCESSING
The SELECT OUT pins (6 and 23) enable greater system
design flexibility by providing a means to implement an ex-
ternal processing loop. This loop can be used for noise re-
duction circuits such as DNR (LM1894) or multi-band
graphic equalizers (LMC835). If both are used, it is important
to ensure that the noise reduction circuitry precede the
equalization circuits. Failure to do so results in improper op-
eration of the noise reduction circuits. The system shown in
Figure 3 utilizes the external loop to include DNR and a
multi-band equalizer.
TONE CONTROL RESPONSE
Bass and treble tone controls are included in the LMC1982.
The tone controls used just two external capacitors for each
stereo channel. Each has a corner frequency determined by
the value of C2 and C3 (see Figure 4) and internal resistors
in the feedback loop of the internal tone amplifier. The
maximum-boost or cut is determined by the data sent to the
LMC1982 (see Table 1).
The typical tone control response shown in Typical Perfor-
mance Curves were generated with C2 = C3 = 0.0082 µF
and show the response for each step. When modifying the
tone control response it is important to note that the ratio of
C3 and C2 sets the mid-frequency gain. Symmetrical tone
response is achieved when C2 = C3. However, with
C2 = 2(C3) and the tone controls set to “flat”, the frequency
response will be flat at 20 Hz and 20 kHz, and +6 dB at
1 kHz.
The frequency where a tone control begins to deviate from a
flat response is referred to as the turn-over frequency. With
C = C2 = C3, the LMC1982’s treble turn-over frequency is
nominally
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